Story of the Slide

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STORY OF THE SLIDE Tragedy in Two Parts Mark Nowlin of the Seattle Times created this illustration showing the two phases of the deadly slide. The Times later won a Pulitzer for its reporting on the event. [Photograph, upper right: bare grey slide scarp above mounded debris and downed logs, forested ridge behind. Credit in white at its lower left:] Photo by Jake_T53 How the March 22 slide unfolded Geologic and seismic evidence shows the Oso slide occurred in two phases. [Seismic trace chart, upper right of the illustration:] Seismic signals as recorded at the closest seismic station, about 7 miles southwest of the Oso slide area. FIRST PHASE SECOND PHASE 800 600 400 200 0 -200 -400 -600 -800 -1,000 10:39 a.m. 10:42 a.m. [First of three cross-section drawings; left-hand strata labels, then callouts:] SAND AND GRAVEL MIXTURE OF CLAY, SILT, SAND AND GRAVEL SAND SILT/CLAY MIX Ancient scarp 2006 slide scarp Churned-up material from recent slides SLOPE Churned-up material from older slides TOE Stillaguamish River [Second cross-section drawing:] ① ② ③ Ancient scarp FIRST PHASE 1. Groundwater saturated the soil in the lower part of the slope, a mixture of clay, sand, gravel and silt. The soil collapsed and slid as a liquefied debris flow. Duration: 2.3 minutes START: 10:37 a.m. Churned-up material from older slides 2. The debris flow hit the Stillaguamish River, shoving the water ahead of it. 3. Everything in frontof [sic] the flow was smashed and swept forward as the flow traveled 0.7 miles. [Third cross-section drawing:] Ancient scarp SECOND PHASE After the lower slope slid away, the upper slope failed and collapsed in a second massive slide that did not travel as far. START: 10:41 a.m. Duration: 2.16 minutes NEW SLOPE New river channel First phase debris flow NEW TOE Source: Pacific Northwest Seismic Network MARK NOWLIN / THE SEATTLE TIMES

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